U.S. Pat. No. 5,616,266, entitled "Resistance Heating Element with Large Area, Thin Film and Method," issued Apr. 1, 1997 and co-pending patent application, Ser. No. 08/874,524, entitled "Method and Apparatus for Edge Heating of Thin Film Heating Element," filed Jun. 13, 1997, both assigned to assignee of the present patent application, disclose thin film resistance heating elements for use in a variety of oven and space heater applications. The present invention improves upon the design of the thin film heaters disclosed in these patents.
The '266 patent discloses a thin film heater having a metal substrate with a ceramic layer thermally bonded across one side of the metal substrate. An electrically conductive, large area thin metallic film is deposited on the ceramic layer, isolated from the metal substrate. A pair of spaced apart electrical terminals are provided at the ends of the conductive film. Preferably, the conductive film is stannic oxide (tin-oxide) and is deposited onto the ceramic layer as a very thin film of, for example, 2 microns or less. Large area heaters constructed in this manner have been found to be capable of temperatures in excess of 500.degree. F. while allowing operation at high power levels, but lower power densities. Low power densities produce an extremely even heat at lower temperatures without significant hot spots or excessive thermal gradients over the area of the panel.
Co-pending application Ser. No. 08/874,524 discloses a method and apparatus for controlling heat loss at the peripheral edges of heaters of the type disclosed in the '266 patent. A thin film conductive edge heater strip is formed around the peripheral edges of a large area thin film heater and is separately controlled to adjust its heat loss in order to compensate for heat loss at the outer edges of the large area heater. This design can be used in combination with the improved thin film heater of the present invention.
Circular heating elements are conventionally made in the form of a spiral, such as the spiral heating elements of electric stove-top heaters. Circular heaters are employed because most cooking utensils are circular and because a round shape improves the efficiency of the heater by matching more closely the geometry of the load. Thick film heaters, typically 0.001" thick, provide a relatively uniform, low-temperature operating surface. However, a more uniform and lower operating temperature heater is the thin film heater, such as disclosed in the '266 patent. Thin film heaters made from metal oxides, such as tin-oxide, provide long term durability and stability up to approximately 500.degree. C., which is hot enough not only for cooking purposes but for many other purposes as well.